1. Field of the Invention
The present invention relates generally to an apparatus with a plurality of contacts for connecting and disconnecting an electrical power source to an electrical device while suppressing transient energy between the plurality of contacts and more particularly to an electrical contactor electrically coupled to a positive temperature coefficient resistivity (PTC) element for reducing arcing between the electrical contactor's contacts.
2. Description of the Related Art
FIGS. 1(a) and 1(b) illustrate a prior art electrical contactor 10, which is commonly used as a control device for various types of motors. The contactor 10 has a yoke 12, comprising a ferromagnetic material, and at least one coil 14, which is positioned in at least one opening in yoke 12. An armature 16, also comprising a ferromagnetic material, is positioned atop yoke 12 and can be manipulated by at least one coil 14, as discussed below.
Two pairs of contacts 18, 20 and 22, 24 are typically used in electrical contactor 10. Contacts 18, 22 are stationary, with contact 18 mounted on line terminal 26 and contact 22 mounted on load terminal 28. Contacts 20, 24 are mounted on blade 30, which moves in tandem with armature 16. Armature 16 and blade 30 are affixed to a shaft or carrier 32, having a distal end and a proximal end, and comprising a nonconductive material, for example, a thermoplastic. Carrier 32 may contain a recess for receipt of a compression spring 34, which is used to ensure contact between contacts 18, 20, 22 and 24. The electrical contactor 10 contains additional components which are well known in the art and, therefore, these components have not been discussed for the sake of simplicity.
FIG. 1(a) illustrates an electrical contactor 10 in an “open” or non-conducting position. In the open position, no current flows though coil 14. Subsequently, there is no electromagnetic force interacting with armature 16.
As current flows through coil 14, an electromagnetic force is generated which attracts armature 16. Armature 16 is pulled toward yoke 12 and coil 14 by the electromagnetic force, which causes carrier 32 and blade 30 to move toward yoke 12 and coil 14. As the armature 16, carrier 32 and blade 30 move toward the yoke 12 and coil 14, moveable contact 20 contacts stationary contact 18 and moveable contact 24 contacts stationary contact 22, as illustrated in FIG. 1(b). Contact between the two pairs of contacts 18, 20, 22 and 24 is substantially simultaneous. As discussed above, compression spring 34 places a sufficient force on carrier 32 to ensure that moveable contacts 20, 24 contact stationary contacts 18, 22, respectively, when current flows though coil 14.
Upon termination or interruption of current flow in coil 14 and removal of the electromagnetic force, armature 16, carrier 32 and blade 30 move away from yoke 12 and coil 14 and moveable contacts 20, 24 separate from stationary contacts 18, 22, respectively, as illustrated in FIG. 1(a).
There are several disadvantages with use of the prior art electrical contactor 10. Arching may occur when contacts 18, 20, 22 and 24 close and separate, thereby gradually eroding away the contacts. Severe arc erosion commonly occurs during motor startup or interruption of a locked motor. Typically, existing electrical contactors are limited to a maximum of approximately 9,000 operation cycles under such severe conditions because the electrical contactors have to make or interrupt approximately 6 times the contactor's continuous current rating.
Another disadvantage of the prior art is contact welding. With the existence of high currents during startup of a motor or interruption of a locked motor, contacts 18, 20, 22 and 24 are prone to become welded together resulting in a permanent closed circuit. Welding of the contacts 18, 20, 22 and 24 may lead an operator to abuse the on/off mechanism and/or contactor case, further damaging the device in an attempt to force open the contacts of the electrical contactor 10. Additionally, the welds may be broken by operation of the contactor following a welding of the contacts. In this case, severe pitting may exist in the vicinity of the broken weld. This may lead to improper contact engagement during future operations resulting in a high resistance contact, which may further lead to decreased contact life, additional welding or a runaway thermal condition. Clearly, once contact welding occurs, the electrical contactor will fail to function properly.
In an attempt to reduce the possibility of arc welding of the contacts, silver/cadmium oxide contacts are commonly used. Cadmium is a harmful element to human beings and the United States Environmental Protection Agency (EPA) has expressed concern about the use of silver/cadmium oxide contacts. Therefore, an alternative to the use of silver/cadmium oxide contacts is desirable.
Thus there is a need for a simple electrical contactor which overcomes the foregoing disadvantages of the prior art by providing a positive temperature coefficient resistivity element electrically coupled or connected to a plurality of contacts in the electrical contactor to suppress transient energy between the contacts.